This invention relates to emulsion polymerization and more particularly to the preparation of large-particle-size monodisperse latexes by seeded emulsion ploymerization in microgravity.
Many applications exist for chemically inert polymeric spherical particles having a uniform size in the multimicron range. Such particles are utilized in the form of a monodisperse latex; "monodisperse" being defined as having a standard deviation of particle size of one percent or less and "latex" being defined as a stable emulsion made up of a polymeric material dispersed in an aqueous continuous phase. Monodisperse latexes have been used for calibration of various instruments, the known uniform size of the polymer particles providing a reference standard. In the field of medical research, monodisperse latexes provide a useful tool for measurement of pore sizes in body membranes. For example latex particles 2 microns in diameter have been used in studies to determine the tendency of foreign particles to penetrate the pores of intestinal walls and enter body tissue. Serological diagnostic tests for disease such as rheumatoid arthritis also utilize monodisperse latexes. Numerous other applications are awaiting the availability of monodisperse latexes at particle sizes of interest above 2 microns.
Monodisperse latexes having particles up to approximately 2 microns in diameter have been prepared by seeded emulsion polymerization in practical quantities, particles as large as 5.6 microns in 100 gram quantities and particles as large as 12 microns in microscopic quantities, but no method exists for preparation of monodisperse particles in practical quantities in the size range over 2 microns. In existing processes for preparing the 2 micron and smaller latexes a monodisperse latex having a very small particle size of several tenths of a micron is first prepared from the monomer and is then used as "seed" for growing larger particles. The initial seed particle preparation step is well known, and relatively easy to perform. However, particle growing steps become increasingly difficult with larger particle sizes, and a practical maximum is reached at about 2 microns.
Difficulties with growing large-particle-size (over 2 micron) latexes are believed to result from exceeding the size range in which colloidial and surface properties determine the behavior of the system and entering the range in which bulk properties are determinitative. At relatively small particle sizes (0.2 to 0.4 micron) the particle growth process can be readily controlled by use of an amount of emulsifier large enough to prevent coagulation but small enough to avoid formation of new particles. The emulsifier concentration range that gives no coagulum and no new particles is relatively broad. However, as the particles are grown to larger and larger sizes in successive seeding steps it becomes increasingly difficult to maintain a stable, uncoagulated emulsion without forming new particles and thereby destroying monodispersity of the latex. The effective range of emulsifier concentration becomes more and more narrow until at 1 to 2 microns the reaction becomes unpredictable; duplicate polymerizations may give either a relatively unstable (during polymerization) monodisperse latex or a stable latex with a crop of new particles. At sizes over 2 microns the process is virtually inoperative or at best effective only for very small quantities and not amenable to scale-up.
The principal reason for instability of large-particle-size monodisperse latexes is the tendency of particles to settle or cream upon standing. The critical size for settling in the case of polystyrene particles (density 1.050 gm/cc) in water is 0.65 micron. It can be proven experimentally that polystyrene particles of 0.8 micron or larger diameter slowly settle out upon standing, while particles of 0.5 or smaller diameter never settle out. Therefore, the larger the particle size of the latex, the greater the tendency to settle. This tendency can be offset by agitation, and almost all emulsion polymerizations are stirred more or less rapidly to provide mixing the ingredients and good heat transfer. However, too great a stirring rate can also give irreversible coagulation of the latex particles, particularly if they are swollen with monomer, as they are during the early stages of the reaction.
The theory of coagulation of colloidial sols can be divided into two classifications: diffusion-controlled and agitation-controlled flocculation. In a given case, both mechanisms are operative but in general diffusion-controlled flocculation is predominant at particle sizes of approximately 0.1 micron, while at about 1.0 micron each mechanism is equally operative, and at sizes much larger than 1.0 micron agitation-induced flocculation is predominant. This means that in a stirred system, the formation of coagulum by flocculation of the particles to form relatively small aggregates proceeds by diffusion until the aggregates reach about 1 micron in size, after which their growth becomes autoaccelerating and they quickly become very large. Thus the tendency to form coagulum by agitation as well as the tendency to settle increases with increasing particle size.
One means of alleviating the tendency to settle and thus facilitating preparation of larger sized monodisperse particles is to go to a lower density polymeric material. Polyvinyltoluene, for example has a density of 1.027 and is thus less likely than polystyrene to settle in water. While the use of a polymer having a density near the density of water is of some assistance in preventing flocculation, the process is still impractical at sizes over 2 microns owing to creaming that results from density differences between polymer and monomer. This difference causes the density of the growing latex particles to increase during polymerization and results in a tendency of the polymer particles, which are swollen with low-density monomer, to cream during the first stages of polymerization and settle in the later stages. Changes in density during the course of polymerization thus render impossible a perfect matching of particle density with water density.